1. Field of the Invention
The present invention relates to a method and system for transmitting a unique signal to a remote device and implementing a respective command, and more particularly, wherein the signal contains both fixed frequency and modulated frequency components which enhance the security of the command implementation.
2. Description of the Related Art
The present invention relates to techniques to prevent duplication of a signal utilized to communicate an implementation command to provide remote vehicle access as well as to remotely control various functions onboard a vehicle. Vehicle access codes used to accomplish such communication are susceptible to unauthorized recordation and duplication. That is, the access code of a transmitter can be duplicated by unauthorized individuals permitting unauthorized access to the vehicle. Such is especially prevalent in radio frequency remote vehicle access devices as recordation can be made with conventional magnetic tape recording devices which may then readily reproduce the signal as desired permitting unauthorized access.
An approach to prevent such duplication is to implement a rolling code. In such a system, the transmitter and receiver each advance to identical new codes each time the unit is used. The receiver ignores previously acceptable codes. Therefore, if a code is recorded by an unauthorized individual, the next time the system is used, the receiver has changed to a new code ignoring the sequentially prior code. The increased security provided by use of a rolling code comes at the cost of some inconvenience. That is, the rolling code in the transmitter and the receiver may not stay synchronized, as the transmitter may be actuated beyond the range of the receiver resulting in an increment or roll of the code stored in the transmitter without a corresponding roll of the receiver code. In this case, if the transmitter is within the look ahead range of the receiver, the receiver code will be advanced until a match occurs. However, if the transmitter is beyond the look ahead range, user action will be required. Therein lies the inconvenience as complicated techniques must be utilized to resynchronize the transmitter and receiver pair. In addition, such resynchronization techniques are required when the transmitter battery is charged.
Additionally, there are techniques to accomplish friend/foe detection by use of multiple frequency signals. One of such techniques involves superimposing the command with a jamming signal. Another technique utilizes two signals which appear in a predetermined sequence to establish a friend/foe screen. The screen is passed only if the two signals are detected thus opening a window through which a subsequently received command containing a unique code is received thereby implementing the respective command. Each of these dual signal techniques require use of systems capable of transmitting and receiving dual signals which results in added cost. Therefore, there is a need for a low cost uncomplicated system to achieve a secure remote vehicle access.
In communicating a command from a remote handheld transmitter to a fixed receiver on-board a vehicle, an oscillator is typically used to generate a radio frequency signal. Receivers utilized for such purposes have fixed band widths, whereas remotely operable handheld oscillators are typically not stable due to the variety of operating conditions and manufacturing tolerances encountered. When such an oscillator wanders outside the band width of the receiver, the received signal strength diminishes and ultimately fails. Stability may be achieved by incorporating a stabilizer in the oscillator circuit. A transmitter incorporating a stabilizer may then be utilized to generate a pulse train which communicates a code to implement a command. Such a pulse train is easily copied and reproduced permitting unauthorized access to the vehicle. An object of the present invention is to impede the ability to copy such signals in a low cost system.